Strengthening of the spokes against flexure may be achieved by physically supporting the spokes, as with a ring adhesively connected to the spokes near the hub. Such physical additions constitute added manufacturing and materials expenses, and may unduly affect the resistance of the spokes to flexure in actual operation.
U.S. Pat. No. 4,018,639 to Staples, Jr. is illustrative of such a printhead in which the spokes are one, flexible material and the impact face is another material resistant to impact. The impact faces are separate elements or slugs which are attached to the spokes. The slugs are molded into the ends of the printhead spokes during manufacture. This printhead also has rings adhesively attached to alter the deflection properties of the spokes.
U.S. Pat. No. 3,935,937 to Tramposch similarly discloses detached character slugs. U.S. Pat. No. 4,165,189 to Bauer shows a molded daisywheel printhead in which the spokes are reinforced by ribs formed in them.
Two-shot molding in another known method of manufacturing printwheels with two different materials in the desired locations. Several commercially-sold printwheels appear to be made by two-shot molding, typically having a very hard material for the character faces. This method involves separate molds for injection of each material. After the first material is molded, (the character faces, for example) the resulting part is removed from the mold and inserted into a second mold. The second material is injected into this mold, forming the remaining sections of the printwheel.
This printwheel employs molding, but does not involve changing the form of the spokes. It is conventional to mold a daisywheel printhead in one piece, and the printhead in accordance with this invention is molded in one piece. This invention differs from known prior art in that the printhead is molded entirely from flowed material molded in place, but of materials of different characteristics. The outer material of the lower spokes is primarily resistant to flexure while the entire material of the outer impact faces is resistant to impact.
This is achieved by coinjection molding. Coinjection molding is standard in itself. It is used to achieve a product with a core of one material and an outer layer of another. In accordance with this invention, one of two different materials coninjected does not reach the outer areas of the mold, where the impact faces are formed. Coinjection in which one material does not reach the ends of the mold occurs in the prior art, but is considered a failure or is done for visual or cosmetic purposes. Similarly, injection molding is a standard process for manufacture of daisywheels. The impact-face material of the preferred embodiment of this invention is a mineral-fiber filled nylon which is the same material as that of printwheels widely sold by the assignee of this invention as the only material of daisywheel. This existing printwheel also has an adhesively attached ring as mentioned above, and it is to be mounted in a cartridge, which, of course, is not integrally molded with the daisywheel.
Injection molding two materials for visual or cosmetic purposes, as mentioned above, may be by interval injection molding (injecting in alternating sequence) or two component injection molding (injecting simultaneously with little control of the flow path of the materials, typically to obtain a somewhat random mixture). These are coinjected since different materials are injected from two injection units. Known examples include products in which only one of the injected materials reaches the outer portions of the final article. The purpose is to have an article with different colors, one known example being a leaf-shaped article with an internal brown material and an outer green material. This does not involve significant modification of mechanical properties and clearly does not suggest a coinjected daisywheel printhead with materials selected for their mechanical properties. In its more specific aspects, this invention involves coinjection molding of one material as an outer layer around a different material, except inner material extends out to the impact faces and is the only material at the impct faces. Two-component injection and interval injection molding typically do not involve a product having an outer layer around an inner material, and are not known to have been used to make an impact print element such as a daisywheel.